1. Field of the Invention
The present invention relates to a pneumatic tire for passenger cars, and more particularly to a pneumatic tire for passenger cars which has excellent durability and can be safely used even when the internal pressure of the tire is reduced.
2. Description of the Related Art
Cords of aliphatic polyamide fibers such as nylon 66 fiber are frequently used as reinforcing cords for tires because of their high strength and excellent resistance to wear. However, these cords do not always give satisfactory results when they are used for pneumatic tires as passenger cars.
Pneumatic tires for passenger cars have been improved remarkably in recent years by the use of cords of polyester fibers which show excellent dimensional stability and have high strength in place of the above fibers for reinforcement of the tires.
Among the polyester fibers, polyethylene terephthalate fibers (PET) are generally used as the reinforcing material for rubber such as tire cords because PET has a Young's modulus higher than that of nylon 66 fiber and about the same as that of rayon and shows a better balance of physical properties than other fiber materials.
However, PET exhibits inferior dimensional stability under heating because of a decrease in the Young's modulus although PET has a relatively high Young's modulus at room temperature. Therefore, in spite of the high Young's modulus, PET manufactured to suit industrial applications such as the reinforcement of rubber has a drawback in that the Young's modulus decreases under heating to about the same value as that of general use PET used for clothing.
Polyethylene naphthalate fibers (PEN) stretched sufficiently by a heat treatment show a Young's modulus almost twice that of PET at room temperature. Moreover, PEN has a Young's modulus as high as 100 g/d or more at 100.degree. C. or higher and exhibits excellent dimensional stability under heating such as a dry heat shrinkage of 2% or less at 150.degree. C. Therefore, the application of PEN as the reinforcing material of rubber such as tire cords has been attempted.
However, heat buildup inside a tire is extremely large when the tire is used with a lower internal pressure, i.e., under so-called run-flat conditions. Therefore, adhesion at high temperatures is not sufficient even when PET or PEN is used. The main cause of failure of a tire during use in a run-flat condition is separation at the interface of the PET or PEN and the adhesive layers.